1. Field of the Invention
The present invention relates generally to a protective circuit for a switching regulator, and is directed more particularly to a protective circuit for a switching regulator by which a transformer and so on used therein are effectively prevented from being damaged by the temperature increase in the transformer.
2. Description of the Prior Art
In the art there has been proposed a protective circuit for a switching regulator in which a special thermal fuse is provided so as to protect a transformer used therein from being damaged by the temperature increase thereof as shown in FIG. 1. In the figure, 1 designates a DC power source provided by rectifying a commercial power source, for example, a battery whose positive electrode is connected through a special thermal fuse 2, which is molten and hence cut off at a predetermined temperature, to one end of a primary winding 3a of a transformer 3. In this case, the thermal fuse 2 is located near the transformer 3, i.e., its core. The other end of the primary winding 3a is connected to the collector of an NPN-type transistor 4 which serves as a switching element. The switching transistor 4 is connected at its emitter to the negative electrode of the battery 1 and at its base to the output side of a pulse width modulator 5. A secondary winding 3b of the transformer 3 is connected through a regulator circuit 6 to a load 7. The output side of the regulator circuit 6 is connected to one of the input terminals of an error voltage detector circuit 8 which is supplied at its other input terminal with a reference voltage from a reference voltage source 9. An error signal appears at the output side of the error voltage detector 8 is applied through an isolation device or isolator 10 such as a hot-coupler, which serves to isolate between the primary and secondary sides of the transformer 3, to the pulse width modulator 5 as its pulse width control signal. Thus, the pulse width of a pulse developed at the output side of the pulse width modulator 5 is controlled, then the ON-time of the transistor 4 is controlled, and hence a DC voltage developed at the output side of the regulator 6 can be made constant.
In the above stable power supply circuit of a switching system, when heat is generated in the transformer 3 and the temperature of the transformer 3 becomes high to damage the transformer 3 itself and other circuit elements, the thermal fuse 2 is molten and then cut to protect the transformer 3.
This stable power supply circuit is of meritorious value, that the transformer 3 can be made small in size and hence the whole circuit becomes compact. The special thermal fuse 2 is large in size, which makes the whole circuit large, and the fuse 2 must be exchanged with a new one at every time when it is cut, with the result that the merit of the stable power supply circuit of switching system is reduced a great deal.